Reduce public surface area of ColumnType and family. (#1543)

* Reduce public surface area of ColumnType and family.

* Introduce internals-visible-to on core, and [BestFriend] attributes on key
  members of ColumnType so internally the implementation can use the old
  conveniences.

* Make type-specific data available only on the relevant type, for example,
  Size on VectorType, which replaces all of VectorSize, ValueCount,
  IsKnownSizeVectorType.

* All `IsX` and `AsX` should be replaced with `is XType` or `as XType`.

* Also in the spirit of the above, hide other redundant conveniences.

* De-emphasize rather entirely DataKind as having anything to do with
  ColumnType, at least publicly.

* Validate new public surface by having the tests use it instead of the old
  way.

Author: TomFinley

src/Microsoft.ML.Api/CodeGenerationUtils.cs

@@ -75,11 +75,11 @@ public static void AppendFieldDeclaration(CSharpCodeProvider codeProvider, Strin
             target.Append(indent);
             target.AppendFormat("public {0} {1}", generatedCsTypeName, fieldName);
 
-            if (appendInitializer && colType.IsKnownSizeVector && !useVBuffer)
+            if (appendInitializer && colType is VectorType vecColType && vecColType.Size > 0 && !useVBuffer)
             {
                 Contracts.Assert(generatedCsTypeName.EndsWith("[]"));
                 var csItemType = generatedCsTypeName.Substring(0, generatedCsTypeName.Length - 2);
-                target.AppendFormat(" = new {0}[{1}]", csItemType, colType.VectorSize);
+                target.AppendFormat(" = new {0}[{1}]", csItemType, vecColType.Size);
             }
             target.AppendLine(";");
         }
@@ -109,17 +109,13 @@ private static string GetBackingTypeName(ColumnType colType, bool useVBuffer, Li
         {
             Contracts.AssertValue(colType);
             Contracts.AssertValue(attributes);
-            if (colType.IsVector)
+            if (colType is VectorType vecColType)
             {
-                if (colType.IsKnownSizeVector)
+                if (vecColType.Size > 0)
                 {
                     // By default, arrays are assumed variable length, unless a [VectorType(dim1, dim2, ...)]
                     // attribute is applied to the fields.
-                    var vectorType = colType.AsVector;
-                    var dimensions = new int[vectorType.DimCount];
-                    for (int i = 0; i < dimensions.Length; i++)
-                        dimensions[i] = vectorType.GetDim(i);
-                    attributes.Add(string.Format("[VectorType({0})]", string.Join(", ", dimensions)));
+                    attributes.Add(string.Format("[VectorType({0})]", string.Join(", ", vecColType.Dimensions)));
                 }
 
                 var itemType = GetBackingTypeName(colType.ItemType, false, attributes);